In recent years, there has been demand for increase in information density of an optical information storage medium at the recording and playback thereof so that an enormous amount of information, such as video, is processed at high speed. In view of this, a super resolution technology has been proposed in which information is recorded in the form of a row of recording marks or prepits, having a shortest mark length shorter than an optical system resolution limit (hereinafter simply referred to as “resolution limit”) of a playback apparatus, and the recorded information is reproduced.
Note that the prepit herein is made up of a concavity and/or a convexity. Further, an optical information storage medium that can be played back by the super resolution technology is referred to as “super resolution medium” or “super resolution optical information storage medium”. An optical information storage medium that can be played back without use of the super resolution technology, i.e. an optical information storage medium where information is recorded in the form of a row of recording marks or prepits, having a shortest mark length longer than a resolution limit of a playback apparatus is referred to as “normal medium” or “normal optical information storage medium”. Note that the resolution limit, which is determined by a wavelength λ of reproduction laser light of the playback apparatus and a numerical aperture NA of an objective lens of the playback apparatus, is theoretically represented by λ/4NA.
Especially desired is the development of a super resolution technology which is applied mainly to a read-only optical information storage medium and in which information is recorded in the form of a row of prepits, having a shortest mark length shorter than the resolution limit of the playback apparatus and the recorded information is reproduced.
Note that the mark length herein refers to a length of a prepit in a track direction and/or a distance between adjacent prepits in the track direction. Further, the mark length is nearly an integral multiple of a channel bit length. For example, in a read-only DVD (Digital Versatile Disc), a shortest mark length is 0.4 μm, which is nearly three times longer than the channel bit length, and there are nine varieties of mark lengths. In a read-only CD (Compact Disc), the shortest mark length is 0.83 μm, which is nearly three times longer than the channel bit length, and there are nine varieties of mark lengths. The super resolution technology can be also applied to a recordable optical information storage medium, where there exists a row of prepits, as address pits, each constituted by a concavity and/or a convexity.
Currently, various kinds of super resolution techniques for reproducing the prepit row have been proposed, including a super resolution technique using a thermochromic film and a super resolution technique using a photochromic film.
For example, a super resolution medium disclosed in Patent Literature 1 is such that a thermochromic dye layer, as a mask layer, that changes its optical property, such as transmittance, with temperature is provided on a reproduction-light-striking surface of a reflection layer. Note that the mask layer is a layer that causes super resolution phenomenon, such as pseudo reduction of a laser spot.
In the super resolution medium disclosed in Patent Literature 1, transmittance distribution occurs due to temperature distribution caused by light intensity distribution that occurs in the laser spot on a reproduction layer located near the reproduction-light-striking surface. For example, assume that the reproduction layer is made from a material that increases a transmittance with temperature. In this case, a transmittance in a high-temperature area only increases, which causes pseudo reduction of the laser spot on the surface of the reflection layer. This makes it possible to reproduce a signal corresponding to a prepit row having a shortest mark length shorter than the resolution limit of the playback apparatus.
Incidentally, the super resolution medium generally requires reproduction light having higher power than the normal medium. One of the reasons for this is that a laser spot is reduced in a pseudo manner by utilizing heat generated in a thin film by the high-power reproduction light and/or the amount of the high-power reproduction light. Further, the amount of change in property of the super resolution medium, for example, error rate (error occurrence rate), caused due to change in reproduction setting values (setting values required for reproduction of recorded information), i.e., a reproduction power value, a servo offset value, a spherical aberration correction value, and a tilt is larger than the amount of change in property of the normal medium. One of the reasons for this is that change in heat generated in a thin film by the reproduction light and/or the amount of the reproduction light changes light intensity distribution of the laser spot to be reduced in a pseudo manner.
For the super resolution medium, it is therefore desirable that optimum reproduction setting values are recorded in advance in the medium.
For example, a super resolution medium disclosed in Patent Literature 2 contains information on optimum reproduction power in advance and therefore enables an effective spot diameter to be always kept constant only by focus servo.